This invention relates to an apparatus for vulcanizing or curing elastomeric articles and more particularly to an apparatus for curing pneumatic tires.
The standard process for converting uncured or "green" rubber into a product that will resist heat and cold in addition to having considerable mechanical strength is called vulcanizing or curing. The rubber used in a pneumatic tire is generally prepared for the vulcanization process by adding sulphur and/or other vulcanizing agents such as accelerators to the rubber. Thereafter, the tire is built of various components including a carcass body made up of plies of reinforcement cords buried in rubber. The built tire before vulcanization is known in the art as a green tire.
A process to cure the rubber in the tire includes (1) placing the green tire over a bladder in a tire mold whereafter the (2) green tire is shaped by pressure from a high temperature fluid, such as steam or hot water, for a brief period of time effecting the tire to expand radially outwardly. The curing apparatus is then closed and the curing bladder is further expanded outwardly by the pressurized high temperature fluid, forcing the tire into the mold and subjecting the tire to heat and pressure of a high temperature fluid for a predetermined time.
During the curing period, the pressurized high temperature fluid is provided in the curing bladder until the cure is completed wherein the heat transferred to the tire from high temperature fluids in the bladder is generally called internal curing. Also, high temperature fluid such as steam is provided externally of the tire during the curing cycle which is generally referred to as external cure.
The high temperature fluid is supplied from conventional sources located internal and external of the tire wherein the external heat is transmitted through the mold to the tire. In particular, one conventonal source for the external high temperature fluid is a chamber in the mold, often referred to as a steam chamber. As a result of the material and geometrical properties of the mold and the tire, various parts of a tire receive different amounts of cure during normal curing operations.
The amount of heat transferred to the tire through the mold is directly dependent on the temperature gradient across the mold, the thickness of the mold between the steam chamber and the interface with the tire and the heat transfer characteristics of the material in the mold, in particular the thermal conductivity characteristics of the material. The heat transferred through a mold made of one material can be expressed as: ##EQU1## where k=thermal conductivity characteristics of the material BTU/Ft.-hr-.degree.F. (kcal/m-hr-.degree.C.)
A=cross-sectional area of the mold, Ft.sup.2 (m.sup.2) PA1 T.sub.2 =temperature of the mold at location x.sub.2, .degree.F. (.degree.C.) PA1 T.sub.1 =temperature of the mold at location x.sub.1, .degree.F. (.degree.C.) PA1 x.sub.2 =distance coordinate of heat energy, Ft (m) PA1 x.sub.1 =distance coordinate of heat energy, Ft (m)
The heat transferred through a mold as expressed in equation (1) is directly proportional to the value of the thermal conductivity, k, of the material in the mold. Therefore, the higher the value of the thermal conductivity of the material, the more heat that will be transferred from the heat source to the tire. The thermal conductivity of aluminum, a material generally used in molds, is approximately 120 BTU/Ft.-hr-.degree.F. at 212.degree. F. (178.8 kcal/m-hr-.degree.C. at 100 C..degree.) whereas the thermal conductivity of a mold made of steel is about 20 to about 30 BTU/Ft-hr-.degree.F. at 212.degree. F. (about 29.8 to about 44.8 kcal/m-hr-.degree.C. at 100.degree. C.) whereas the thermal conductivity through air at 212.degree. F. (100.degree. C.) is approximately 0.017 BTU/Ft-hr-.degree.F. at 212.degree. F. (0.025 kcal/m-hr-.degree.C. at 100 C..degree.).
In the mold containing a mold back and a replaceable tread ring, the heat flows from the steam chamber through the mold back and through the tread ring into the tire. The amount of heat transferred in this mold will be affected by the thermal conductivity of the materials in both the mold back and tread ring. In a conventional mold, the materials in the mold back and tread ring are similar, resulting in an assumption that the heat flow is similar to that in a mold where there is no replaceable tread ring (i.e., the mold back and tread ring are one) assuming the fit between the mold back and tread is tight enough to exclude air.
The rubber compositions in the various parts of a tire require different amounts of cure and/or different temperatures for optimum property development. For example, the temperature required in the tread area of a tire may be less than the temperature desired for the bead area as found in low rolling resistance tires. It has been found in particular in low rolling resistance tires that temperatures in the range of about 300.degree. F. (149.degree. C.) to about 320.degree. F. (160.degree. C.) applied to the tread area result in undesirable increased rolling resistance of the rubber in the tread area whereas a temperature in the range of about 280.degree. F. (138.degree. C.) to about 290.degree. F. (143.degree. C.) result in the desirable low rolling resistance of the rubber in the tread area. However, reduction of temperature of the external cure temperature to about 280.degree. F. (138.degree. C.) to about 290.degree. F. (143.degree. C.) has resulted in undercuring in the bead area of a low rolling resistance tire.
Presently, the requirement of lower temperature in areas of the tire, and in particular in the tread area, is accomplished by reducing the temperature of the external high temperature fluid. Associated with this lower temperature is a production loss because of the overall longer time required to cure the complete tire.